Various central nervous system disorders such as anxiety, depression, motor disorders etc., are believed to involve a disturbance of the neurotransmitter 5-hydroxytryptamine (5-HT) or serotonin. Serotonin is localized in the central and peripheral nervous systems and is known to affect many types of conditions including psychiatric disorders, motor activity, feeding behavior, sexual activity and neuroendocrine regulation among others. 5-HT receptor subtypes regulate the various effects of serotonin. Known 5-HT receptor family includes the 5-HT family (e.g. 5-HT1A), the 5-HT2 family (e.g. 5-HT2A & 5-HT2C), 5-HT3, 5-HT4, 5-HT5, 5-HT6 and 5-HT7 subtypes.
The 5-HT6 receptor subtype was first cloned from rat tissue in 1993 (Monsma, F. J.; Shen, Y.; Ward, R. P.; Hamblin, M. W., Sibley, D. R., Molecular Pharmacology, 1993, 43, 320-327) and subsequently from human tissue (Kohen, R.; Metcalf, M. A.; Khan, N.; Druck, T.; Huebner, K.; Sibley, D. R., Journal of Neurochemistry, 1996, 66, 47-56). The receptor is a G-protein coupled receptor (GPCR) positively coupled to adenylate cyclase (Ruat, M.; Traiffort, E.; Arrang, J-M.; Tardivel-Lacombe, L.; Diaz, L.; Leurs, R.; Schwartz, J-C., Biochemical Biophysical Research Communications, 1993, 193, 268-276). The receptor is found almost exclusively in the central nervous system (CNS) areas both in rats as well as in humans.
In situ hybridization studies of 5-HT6 receptor in rat brain using mRNA indicate principal localization in the areas of 5-HT projection including striatum, nucleus accumbens, olfactory tubercle and hippocampal formation (Ward, R. P.; Hamblin, M. W.; Lachowicz, J. E.; Hoffman, B. J.; Sibley, D. R.; Dorsa, D. M., Neuroscience, 1995, 64, 1105-1111). Highest levels of 5-HT6 receptor mRNA has been observed in the olfactory tubercle, the striatum, nucleus accumbens and dentate gyrus as well as CA1, CA2 and CA3 regions of the hippocampus. Lower levels of 5-HT6 receptor mRNA were seen in the granular layer of the cerebellum, several diencephalic nuclei, amygdala and in the cortex. Northern blots have revealed that 5-HT6 receptor mRNA appears to be exclusively present in the brain, with little evidence for its presence in peripheral tissues.
Significant efforts are being made to understand the possible role of the 5-HT6 receptor in psychiatry, cognitive dysfunction, motor function and control, memory, mood and the like. The compounds which demonstrate a binding affinity for the 5-HT6 receptor are earnestly sought both as an aid in the study of the 5-HT6 receptor and as potential therapeutic agents in the treatment of central nervous system disorders, for example see Reavill C. and Rogers D. C., Current Opinion in Investigational Drugs, 2001, 2(1): 104-109, Pharma Press Ltd.
Treatment with 5-HT6 receptor antagonists increases seizure threshold in a rat maximal electroconvulsive-shock test [Stean, T. et al. (1999) Anticonvulsant properties of the selective 5-HT6 receptor antagonist SB-271046 in the rat maximal electroshock seizure threshold test. Br. J. Pharmacol. 127, 131P; Routledge, C. et al. (2000) Characterization of SB-271046: a potent, selective and orally active 5-HT6 receptor antagonist. Br. J. Pharmacol. 130, 1606-1612]. Although this indicates that 5-HT6 receptors might regulate seizure threshold, the effect is not as pronounced as that of known anticonvulsant drugs.
Our understanding of the role of 5-HT6 receptor ligands is most advanced in two therapeutic indications, learning and memory deficits and abnormal feeding behaviour, in which this receptor is likely to have a major role to play. The exact role of the 5-HT6 receptor is yet to be established in other CNS indications such as anxiety; although one 5-HT6 agonist has reached Phase I clinical trials recently, the exact role of the receptor is still to be established and is the focus of significant investigation. There are many potential therapeutic uses for 5-HT6 receptor ligands in humans based on direct effects and on indications from available scientific studies. These studies include the localization of the receptor, the affinity of ligands with known in-vivo activity and various animal studies conducted so far. Preferably, antagonist compounds of 5-HT6 receptors are sought after as therapeutic agents.
One potential therapeutic use of modulators of 5-HT6 receptor functions is in the enhancement of cognition and memory in human diseases such as Alzheimer's. The high levels of receptor found in structures such as the forebrain, including the caudate/putamen, hippocampus, nucleus accumbens and cortex suggests a role for the receptor in memory and cognition since these areas are known to play a vital role in memory (Gerard, C.; Martres, M. P.; Lefevre, K.; Miquel, M. C.; Verge, D.; Lanfumey, R.; Doucet, E.; Hamon, M.; El Mestikawy, S., Brain Research, 1997, 746, 207-219). The ability of known 5-HT6 receptor ligands to enhance cholinergic transmission also supports the potential cognition use (Bentley, J. C.; Boursson, A.; Boess, F. G.; Kone, F. C.; Marsden, C. A.; Petit, N.; Sleight, A. J., British Journal of Pharmacology, 1999, 126 (7), 1537-1542).
Studies have found that a known 5-HT6 selective antagonist significantly increased glutamate and aspartate levels in the frontal cortex without elevating the levels of noradrenaline, dopamine or 5-HT. This selective elevation of certain neurochemicals is noted during memory and cognition, strongly suggests a role for 5-HT6 ligands in cognition (Dawson, L. A.; Nguyen, H. Q.; Li, P. British Journal of Pharmacology, 2000, 130 (1), 23-26). Animal studies of memory and learning with a known selective 5-HT6 antagonist has some positive effects (Rogers, D. C.; Hatcher, P. D.; Hagan, J. J. Society of Neuroscience, Abstracts, 2000, 26, 680).
A related potential therapeutic use for 5-HT6 ligands is in the treatment of attention deficit disorders (ADD, also known as Attention Deficit Hyperactivity Disorder or ADHD) in children as well as adults. As 5-HT6 antagonists appear to enhance the activity of the nigrostriatal dopamine pathway and ADHD has been linked to abnormalities in the caudate (Ernst, M; Zametkin, A. J.; Matochik, J. H.; Jons, P. A.; Cohen, R. M., Journal of Neuroscience, 1998, 18(15), 5901-5907), 5-HT6 antagonists may attenuate attention deficit disorders.
At present, a few fully selective agonists are available. The Wyeth agonist WAY-181187 is currently in Phase I trials to target anxiety [Cole, D. C. et al. (2005) Discovery of a potent, selective and orally active 5-HT6 receptor agonist, WAY-181187. 230th ACS Natl. Meet. (August 28-September 1, Washington D.C.), Abstract MEDI 17.]
International Patent Publication WO 03/066056 A1 reports that antagonism of 5-HT6 receptor could promote neuronal growth within the central nervous system of a mammal. Another International Patent Publication WO 03/065046 A2 discloses new variant of human 5-HT6 receptor and proposes that 5-HT6 receptor is associated with numerous other disorders.
Early studies examining the affinity of various CNS ligands with known therapeutic utility or a strong structural resemblance to known drugs suggests a role for 5-HT6 ligands in the treatment of schizophrenia and depression. For example, clozapine (an effective clinical antipsychotic) has high affinity for the 5-HT6 receptor subtype. Also, several clinical antidepressants have high affinity for the receptor as well and act as antagonists at this site (Branchek, T. A.; Blackburn, T. P., Annual Reviews in Pharmacology and Toxicology, 2000, 40, 319-334).
Further, recent in-vivo studies in rats indicate that 5-HT6 modulators may be useful in the treatment of movement disorders including epilepsy (Stean, T.; Routledge, C.; Upton, N., British Journal of Pharmacology, 1999, 127 Proc. Supplement-131P; and Routledge, C.; Bromidge, S. M.; Moss, S. F.; Price, G. W.; Hirst, W.; Newman, H.; Riley, G.; Gager, T.; Stean, T.; Upton, N.; Clarke, S. E.; Brown, A. M.; British Journal of Pharmacology, 2000, 30 (7), 1606-1612).
Taken together, the above studies strongly suggest that compounds which are 5-HT6 receptor modulators, i.e. ligands, may be useful for therapeutic indications including, the treatment of diseases associated with a deficit in memory, cognition and learning such as Alzheimer's and attention deficit disorder; the treatment of personality disorders such as schizophrenia; the treatment of behavioral disorders, e.g. anxiety, depression and obsessive compulsive disorders; the treatment of motion or motor disorders such as Parkinson's disease and epilepsy; the treatment of diseases associated with neurodegeneration such as stroke or head trauma; or withdrawal from drug addiction including addiction to nicotine, alcohol and other substances of abuse.
Such compounds are also expected to be of use in the treatment of certain gastrointestinal (GI) disorders such as functional bowel disorder. See for example, Roth, B. L.; et al., Journal of Pharmacology and Experimental Therapeutics, 1994, 268, pages 1403-1412; Sibley, D. R.; et al., Molecular Pharmacology, 1993, 43, 320-327.
Furthermore, the effect of 5-HT6 antagonist and 5-HT6 antisense oligonucleotides to reduce food intake in rats has been reported, thus potentially in the treatment of obesity. See for example, Bentley, J. C.; Boursson, A.; Boess, F. G.; Kone, F. C.; Marsden, C. A.; Petit, N.; Sleight, A. J., British Journal of Pharmacology, 1999, 126 (7), 1537-1542); Wooley et al., Neuropharmacology, 2001, 41: 210-129, British Journal of Pharmacology (2006) 1-11, Petrus Johan Pauwels et al and WO 02/098878.
A review by Holenz, Jo″rg et. al., Drug Discovery Today, 11, 7/8, April 2006, Medicinal chemistry strategies to 5-HT6 receptor ligands as potential cognitive enhancers and antiobesity agents, gives elaborate discussion on evolution of 5-HT6 ligands. It had summarized pharmacological tools and preclinical candidates used in evaluation of 5-HT6 receptor in illnesses such as schizophrenia, other dopamine-related disorders and depression and to profile the neurochemical and electrophysiological effects of either blockade or activation of 5-HT6 receptors. Furthermore, they have been used to characterize the 5-HT6 receptor and to investigate its distribution.
So far several clinical candidates form the part of indole-type structures and are closely related structurally to the endogenous ligand 5-HT, for example compounds by Glennon, R. A. et. al., 2-Substituted tryptamines: agents with selectivity for 5-HT6 serotonin receptors, J. Med. Chem. 43, 1011-1018, 2000; Tsai, Y. et. al., N1-(Benzenesulfonyl) tryptamines as novel 5-HT6 antagonists, Bioorg. Med. Chem. Lett. 10, 2295-2299, 2000; Demchyshyn L. et al., ALX-1161: pharmacological properties of a potent and selective 5-HT6 receptor antagonist, 31st Annu. Meet. Soc. Neurosci. (November 10-15), Abstract 266.6, 2001; Slassi, A. et. al., Preparation of 1-(arylsulfonyl)-3-(tetrahydropyridinyl)indoles as 5-HT6 receptor inhibitors, WO 200063203, 2000; Mattsson, C. et. al., Novel, potent and selective 2-alkyl-3-(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole as 5-HT6 receptor agonists, XVIIth International Symposium on Medicinal Chemistry, 2002; Mattsson, C. et. al., 2-Alkyl-3-(1,2,3,6-tetrahydropyridin-4-yl)-1H-indoles as novel 5-HT6 receptor agonists, Bioorg. Med. Chem. Lett. 15, 4230-4234, 2005].
Structure functionality relationships are described in the section on indole-like structures and in a receptor-modeling study in which Pullagurla et. al., claim different binding sites for agonists and antagonists [Pullagurla, M. R. et al. (2004) possible differences in modes of agonist and antagonist binding at human 5-HT6 receptors. Bioorganic Medicinal Chemistry Letters, 14, 4569-4573]. Most antagonists that are reported form part of the monocyclic, bicyclic and tricyclic aryl-piperazine classes [Bromidge, S. M. et. al., (1999) 5-Chloro-N-(4-methoxy-3-piperazin-1-ylphenyl)-3-methyl-2-benzothiophenesulfonamide (SB-271046): A potent, selective and orally bioavailable 5-HT6 receptor antagonist. J. Med. Chem. 42, 202-205; Characterisation of SB-399885, a potent and selective 5-HT6 receptor antagonist. 33rd Annul Meet Society Neuroscience. (November 8-12, New Orleans), Abstract 576.7; Stadler, H. et al. (1999) 5-HT6 antagonists: A novel approach for the symptomatic treatment of Alzheimer's disease. 37th IUPAC Cong. Berlin, Abstract MM-7; Bonhaus, D. W. et al. (2002) Ro-4368554, a high affinity, selective, CNS penetrating 5-HT6 receptor antagonist. 32nd Annu. Meet. Soc. Neurosci., Abstract 884.5.; Beard, C. C. et al. (2002) Preparation of new indole derivatives with 5-HT6 receptor affinity. WO patent 2002098857].
Ro 63-0563: Potent and selective antagonist at human and rat 5-HT6 receptors. Br. J. Pharmacol. 124, (556-562). Phase II antagonist candidate from GlaxoSmithKline, SB-742457 for the therapeutic indication of cognitive dysfunction associated with Alzheimer's disease [Ahmed, M. et al. (2003) Novel compounds. WO patent 2003080580], and the Lilly compound LY-483518 [Filla, S. A. et al. (2002) Preparation of benzenesulfonic acid indol-5-yl esters as antagonists of the 5-HT6 receptor, WO 2002060871]. SB-271046, the first 5-HT6 receptor antagonist to enter Phase I clinical development, has been discontinued (probably because of poor penetration of the blood-brain barrier). In addition, the selective 5-HT6 receptor antagonist SB-271046 is inactive in animal tests related to either positive or negative symptoms of schizophrenia [Pouzet, B. et al. (2002) Effects of the 5-HT6 receptor antagonist, SB-271046, in animal models for schizophrenia. Pharmacol. Biochem. Behav. 71, 635-643].
International Patent Publications WO 2007/046112, WO 2007/020653, WO2007/138611, WO 2005/066157, WO 2004/108671, WO 2004/048331, WO 2004/048330 and WO 2004/048328 (all assigned to Suven Life Sciences Limited) describe the related prior art. Further WO 98/27081, WO 99/02502, WO 99/37623, WO 99/42465 and WO 01/32646 (all assigned to Glaxo SmithKline Beecham PLC) disclose a series of aryl sulfonamide and sulfoxide compounds as 5-HT6 receptor antagonists and are claimed to be useful in the treatment of various CNS disorders. While some 5-HT6 modulators have been disclosed, there continues to be a need for compounds that are useful for modulating 5-HT6. In our research in area of 5-HT6 receptors, we found that sulfone compounds of formula (I) demonstrate very high 5-HT6 receptor affinity. Therefore, it is an object of this invention to provide compounds, which are useful as therapeutic agents in the treatment/prevention of a variety of central nervous system disorders or disorders affected by the 5-HT6 receptor.